The present exemplary embodiment relates to motion control systems. In one embodiment, velocity, acceleration and jerk of a motion control system can be modified. It finds particular application with controlling the jerk parameters of a system in order to eliminate unwanted velocity reversal. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Motion control systems are employed to control motion within a system. A motion control system is generally comprised of a motion controller, a drive, a motor, one or more mechanical elements and a position feedback device. Application software can be employed to command target positions and motion control profiles. The motion controller can act as the intelligence of a system by taking the desired target positions and motion profiles and creating the trajectories for the motors to follow. The drive takes commands from the controller and generates the current required to drive or turn the motor. The motor turns electrical energy into mechanical energy and produces the torque required to move to a desired target position. Motors are designed to provide torque to some mechanics such as linear slides, robotic arms or actuators. The position feedback device is not required for some motion control applications (e.g., stepper motors), but can be employed with others (e.g., servo motors). The feedback device, usually a quadrature encoder, senses the motor position and reports the result to the controller, thereby closing the loop to the motion controller.
The motion controller calculates each commanded move trajectory and further utilizes such calculated trajectories to determine the proper torque command to send the motor drive and actually cause motion. The motion trajectory describes the motion controller board control or command signal output to the driver, resulting in a motor/motion action that follows the profile. The typical motion controller calculates the motion profile trajectory segments based on the parameter values you program. The motion controller uses the desired target position, maximum target velocity, and acceleration values provided to the system to determine how much time it spends in three primary move segments (acceleration, constant velocity and deceleration).
In order to achieve smooth high-speed motion without overtaxing the motor, the controller must direct the motor driver to operate judiciously to achieve optimum results. This is accomplished using shaped velocity profiles to limit the acceleration and deceleration profiles required. Two disparate profiles are commonly employed, a trapezoidal profile and an s-curve profile. A trapezoidal profile changes velocity in a linear fashion until the target velocity is reached. A trapezoidal profile typically results in a shorter duration of motion. When decelerating, the velocity decreases in a linear manner until it reaches a zero velocity. Graphing velocity versus time results in a trapezoidal plot. Advances in technology allow user modification of the acceleration/deceleration with more sophisticated controllers to provide individual settings for various motion parameters. In this manner, trapezoidal motion profiles are employed to obtain higher speeds without skipping steps or stalling.
Although a trapezoidal velocity profile is adequate for most applications, such a profile may cause some system disturbances located at “corners” of the trapezoidal profile. These disturbances can be realized as small vibrations that extend the settling time. For demanding applications sensitive to this phenomenon, the profile can be modified to have an S-shape during the acceleration and deceleration periods. This can minimize the vibrations realized by a device controlled by a motion control system. The S-curve profile can take more time to complete, but a jerk response at the beginning, ending, and transition points are removed. Jerk can be found at the transition points wherein there is acceleration to maximum velocity and maximum velocity to deceleration.
S-curve acceleration and deceleration refers to the shape of the velocity profile of a given move. Without using s-curve acceleration when you load an acceleration, velocity and position, the motor tries to go from zero to the specified acceleration instantaneously. When a motor does this, it creates a trapezoidal velocity profile. When the motor is ready to stop, it once again goes from a zero acceleration to a negative acceleration as fast as it can until it is at a zero velocity and then abruptly stops. These abrupt starts and stops create the sharp corners of a trapezoidal profile. The sharp corners translate to a very high jerk. Jerk is the derivative of acceleration and refers to abrupt changes in acceleration.
The smooth control for a change in the velocity command uses acceleration and deceleration control. Arithmetically, acceleration is the second derivative of position or the first derivative of velocity. A motor is employed to facilitate motion of a load from one location to another. As the motor is at rest, the beginning of the motor profile requests a new velocity from the motor. This instantaneous request for motor velocity requires a lot of energy transfer or “jerk.” Jerk occurs only when a change in acceleration occurs and is defined arithmetically as the derivative of acceleration. Since linear acceleration can be viewed as a ramp, the jerk profile impulses at the beginning and end of the acceleration portions of the move. S-curve acceleration provides a means to soften the jerk. The acceleration rate is first commanded to be low and increased to a maximum rate, then decreased again until the target speed is achieved. This lessens the energy transferred into the load. Two major applications for this type of profiling are to control the shifting of material on the load (but not well fastened) and to prevent positional overshoot in high-inertia loads.
The downside to s-curve profiling is that for a given acceleration time, a higher peak acceleration is required and often requires a larger size motor (more torque) when compared to a linear acceleration profiling. Thus, an s-curve velocity profile can provide smoother motion due to increased setting times. As a result, lower throughput can result. However, since the s-curve reduces required torque at top speed, peak power requirements are reduced.
Conventionally, the user programmed a motion control system with values for initial speed, initial acceleration, maximum velocity, acceleration and deceleration. The corresponding maximum and minimum jerk values were calculated internally and employed by a motion planner through each motion profile. Once a motion profile was initiated, the user witnesses the effect the jerk values had on a system based strictly on empirical evidence. Changes to these values had to be made by estimating correct jerk parameters for each motion profile utilized.
Some motion control systems allow changes to motion parameters (e.g., velocity and acceleration limits) before the previous motion terminates. If an s-curve velocity profile is chosen, the parameter change often results in an undesired velocity profile. Such profiles are typically referred to as velocity reversals, end position overshoots, or velocity runaways. From the user's viewpoint, these behaviors are correct due to jerk rate limitations. The problem is that even experienced users will find it difficult to determine if a parameter change is safe or not. What is needed is a modification of a motion planner to (1) eliminate the above described unwanted velocity reversals, (2) maintain backward compatibility in all other cases, and (3) make its control more intuitive without interfering with the motion planner design.